1. In a medium bowl, whisk
cocoa powder in whipping cream until completely dissolved, smoothing
out any lumps. Add corn syrup, whisk again, and set aside.

2. In a medium heavy-bottomed pot, combine water, sugar, and lime
juice. Over medium-high heat, bring mixture to a boil, stirring
constantly until sugar dissolves. With a pastry brush soaked in water,
brush the sides of the pot clean to remove splatters. Cook without
stirring. As the water evaporates, the bubbles will become bigger and
slower; start watching for caramel color to appear. Swirl or turn the
pot to avoid getting a burned spot and to evenly mix the syrup. Simmer
until mixture is an amber color. Go by the color as the heat will
differ from stove to stove. Don’t allow the caramel to burn and work
carefully so as not to burn yourself.

3. Whisk in whipping cream mixture. The caramel will bubble profusely
and may clump. Keep stirring until the caramel is smooth in color and
texture. Simmer for 2 to 3 minutes or until slightly thickened. Remove
from heat and stir in margarine and vanilla. The caramel will be
extremely hot, so exercise caution as you carefully transfer to a large
bowl and place into the refrigerator. The caramel will thicken as it
cools. To use as a topping, reheat in the microwave until syrupy.